Why Long Distance Runs Are Challenging
When electrical current travels through a conductor, it encounters resistance that causes voltage drop. This voltage loss increases proportionally with distance, making long wire runs one of the most critical considerations in electrical design. A wire size that works perfectly for a 25-foot run may result in unacceptable voltage loss at 150 feet, leading to equipment malfunction, inefficient operation, and potential safety hazards.
Understanding how to properly size wire for long distances is essential for electricians, engineers, and DIY enthusiasts working on projects like detached garages, barn wiring, outdoor lighting, well pumps, and other installations where the load is far from the electrical panel.
Understanding Voltage Drop
What Is Voltage Drop?
Voltage drop is the reduction in electrical potential (voltage) that occurs as current flows through a conductor. Every wire has some electrical resistance, and according to Ohm's Law (V = I × R), when current flows through that resistance, it produces a voltage drop.
Voltage Drop (V) = Current (I) × Resistance (R)
As wire length increases, total resistance increases, resulting in greater voltage drop.
NEC Voltage Drop Recommendations
NEC Guidelines
- Branch circuits: Maximum 3% voltage drop recommended
- Feeders: Maximum 3% voltage drop recommended
- Total (feeder + branch): Maximum 5% total recommended
- Note: These are recommendations, not mandatory code requirements, but are industry best practice
Consequences of Excessive Voltage Drop
- Motor Problems: Motors may overheat, fail to start, or run inefficiently
- Lighting Issues: Lights appear dim or flicker
- Electronic Equipment: Computers and sensitive equipment may malfunction
- Heating Loads: Reduced heating output from resistance heaters
- Energy Waste: Lost power is converted to heat in the wire
Voltage Drop Calculation Methods
Single-Phase Voltage Drop Formula
VD = (2 × K × I × D) / CM
Where K = resistivity constant (12.9 for copper, 21.2 for aluminum), I = current in amps, D = one-way distance in feet, CM = circular mils (wire cross-sectional area)
Three-Phase Voltage Drop Formula
VD = (√3 × K × I × D) / CM
The √3 factor (1.732) accounts for three-phase power delivery.
Calculating Percentage Drop
% Voltage Drop = (VD / Source Voltage) × 100
Example: 7.2V drop on a 240V circuit = 3% voltage drop
Wire Size Reference for Long Runs
The following table shows recommended wire sizes for various loads and distances to maintain approximately 3% voltage drop on 240V circuits:
| Load (Amps) | 50 ft | 100 ft | 150 ft | 200 ft | 300 ft |
|---|---|---|---|---|---|
| 15A | 14 AWG | 12 AWG | 10 AWG | 10 AWG | 8 AWG |
| 20A | 12 AWG | 10 AWG | 8 AWG | 8 AWG | 6 AWG |
| 30A | 10 AWG | 8 AWG | 6 AWG | 6 AWG | 4 AWG |
| 40A | 8 AWG | 6 AWG | 4 AWG | 4 AWG | 2 AWG |
| 50A | 6 AWG | 4 AWG | 3 AWG | 2 AWG | 1 AWG |
| 60A | 6 AWG | 4 AWG | 2 AWG | 1 AWG | 1/0 AWG |
| 100A | 3 AWG | 1 AWG | 1/0 AWG | 2/0 AWG | 3/0 AWG |
Important Note
Practical Long-Run Examples
Example 1: Detached Garage Subpanel
Scenario: 60A subpanel, 150 feet from main panel, 240V single-phase
- Minimum for ampacity: 6 AWG copper
- Voltage drop at 6 AWG: Approximately 5.8% - too high
- Upsized to 4 AWG: Approximately 3.6% - marginal
- Recommended: 2 AWG: Approximately 2.3% - good
In this case, we need to upsize from the minimum 6 AWG required for ampacity to 2 AWG to keep voltage drop under 3%.
Example 2: Well Pump Installation
Scenario: 1.5 HP well pump (10A running, 35A starting), 300 feet from panel, 240V
- Running current consideration: 10A at 300 feet
- Starting current factor: Must handle 35A inrush without excessive drop
- Solution: 6 AWG copper provides acceptable voltage drop for running and starting
Example 3: Barn/Shop Building
Scenario: 100A service, 250 feet from main panel, multiple motor loads
- Minimum for 100A: 3 AWG copper or 1 AWG aluminum
- With voltage drop considered: 2/0 AWG copper or 4/0 AWG aluminum
- Cost consideration: Aluminum may be more economical for this large run
Solutions for Long Distance Runs
1. Upsize the Wire
The most common and reliable solution is simply using larger wire. Each step up in AWG size roughly doubles the cross-sectional area and halves the resistance. The cost of larger wire is usually offset by improved efficiency and reliable operation.
2. Use Higher Voltage
For the same power, higher voltage means lower current, which means less voltage drop. Options include:
- 240V instead of 120V for applicable loads (half the current)
- Three-phase power for large installations
- 480V for industrial applications
3. Consider Aluminum Conductors
For very long runs, aluminum wire can be cost-effective despite needing larger sizes:
- Aluminum costs 60-70% less than copper per pound
- Lighter weight reduces installation labor
- Acceptable for feeders and service entrance conductors
- Requires proper termination techniques and compatible terminals
4. Install a Local Subpanel
Running a larger feeder to a subpanel closer to the loads can be more economical than running multiple circuits:
- One large run instead of multiple smaller runs
- Branch circuit wire runs from subpanel are short
- Easier future expansion
- Better circuit organization and labeling
Installation Tips for Long Runs
Underground Installation
- Use UF-B cable or individual THWN-2 conductors in conduit
- Bury to required depth (18-24 inches depending on protection)
- Include a ground conductor sized per NEC 250.122
- Install an equipment grounding conductor
- Consider pulling string for future additions
Overhead Installation
- Minimum height clearances must be maintained
- Use proper weatherhead and service mast
- Calculate span tension and sag
- Consider triplex cable for simplicity
Common Mistakes to Avoid
Sizing Only for Ampacity
Many installations fail because the wire was sized only for ampacity without considering voltage drop. Always calculate voltage drop for runs over 50 feet.
Ignoring Starting Current
Motors require 3-6 times their running current during startup. Excessive voltage drop during starting can prevent motors from starting or cause them to trip breakers.
Using Wrong K Factor
The resistivity constant K differs between copper (12.9) and aluminum (21.2). Using the wrong value results in incorrect calculations.
Measuring Distance Wrong
Remember that voltage drop calculations use the total circuit length - current flows out AND back. For overhead or underground runs, measure the actual wire path, not straight-line distance.
Planning Tools
Use our free online tools to accurately size wire for your long-distance runs:
- Wire Gauge Calculator - Determine correct wire size for any application
- Voltage Drop Calculator - Calculate exact voltage loss for your run
- AWG Reference Chart - Compare wire specifications
Conclusion
Long distance electrical runs require careful planning to ensure adequate voltage is delivered to the load. By understanding voltage drop calculations and applying appropriate upsizing strategies, you can design installations that operate efficiently and reliably for decades. Remember that the initial cost of larger wire is usually a small fraction of the total project cost and pays for itself through improved efficiency and reliable operation.
Always use our Voltage Drop Calculator to verify your wire sizing, and consult with a licensed electrician for installations involving permits or complex systems.